Such an apparatus including a lens system and intended to manufacture integrated semiconductor circuits is known from U.S. Pat. No. 5,100,237. In this apparatus a mask is illuminated and imaged in a repetitive manner on a photoresist layer provided on a semiconductor substrate. A large number of ICs must be formed on the substrate. After an image of the mask has been formed on the substrate, this substrate is to this end displaced with respect to the mask by a distance which is slightly larger than the length or the width of the ICs to be formed, whereafter a subsequent mask image is made, and so forth. It is desirable that the illumination beam then has an intensity which is as high as possible so that the illumination time for each IC is as short as possible and the time of passage of the substrate through the apparatus, i.e. the time required to illuminate all ICs is as short as possible. Consequently, in addition to a radiation source having a high radiation power, a projection lens system having a high transmission must be used.
To be able to make mask images having very small details, for example with line widths of the order of 0.25 .mu.m, not only very strict requirements should be imposed on the optical quality of the projection lens system, notably on their resolving power, but also the wavelength of the illumination beam should be as small as possible. To this end an illumination system is used which emits radiation at a wavelength in the ultraviolet (UV) range, for example I-line radiation with a wavelength of 365 nm from a mercury vapour lamp or radiation with a wavelength of 243 nm from an excimer laser.
It has been found that the medium within the lens holder is contaminated with organic molecules. These molecules originate from the adhesive with which the lens elements are secured in the holder, of which adhesive a part may evaporate. Moreover, in production circumstances, the ambient air is contaminated with organic molecules which originate for example, from the adhesive layer between the substrate and the photoresist, which molecules may penetrate the lens holder. Although the organic molecules are only present in a small concentration within the lens holder, they may have a disastrous influence on the projection lens system. In fact, such particles may decompose under the influence of the UV illumination beam and subsequently precipitate on the lens elements, forming a carbon or carbon-containing film on these elements so that the effective transmission of these elements will decrease considerably in the course of time.
The projection lens system is a complicated and expensive part of the projection apparatus. The assembly of such a system is complicated and time-consuming. After the system has been assembled, it can no longer be disassembled to clean its components by hand or by mechanical means.
If the medium within the lens holder were air or oxygen, the contaminating organic molecules could be converted via oxidation into carbon dioxide and be depleted by rinsing the lens holder with one of the gases.
However, in said projection lens system a second problem presents itself, viz. due to the exorbitant requirements which are imposed on the optical qualifies of this system, variations of air pressure or temperature play an important role. In fact, such variations cause changes in the difference between the refractive index of the lens elements and the refractive index of the spaces between these elements so that the imaging properties of the projection lens system change, as has been described in the article "Atmospheric pressure induced reduction errors in reduction stepper lenses" in SPIE, Vol. 538, Optical Microlithography IV, 1985, pp. 86-90.
To avoid the second problem it has already been proposed, inter alia in U.S. Pat. No. 4,616,908, to fill the projection lens holder with helium. This gas has a very low refractive index so that the refractive index variations due to atmospheric pressure variations are also much smaller. Since helium is very volatile, fresh gas is to be continuously supplied to the projection lens system so that this system is rinsed, as it were, with helium. Instead of helium, another gas having a low refractive index such as neon or nitrogen can also be used for rinsing. When this type of inert gases is used, the contaminating organic particles cannot be convened and depleted so that the contamination problem occurs.
It is not possible to clean an already contaminated lens system by rinsing it in air or oxygen. It could be considered to clean the contaminated lens system by rinsing it in ozone. However, ozone may attack the cladding layers of the lens elements and the material of these elements themselves so that the optical quality of the projection lens system is reduced again.
The problem of contamination may also occur in other optical apparatuses, for example, those apparatuses in which excimer lasers are used, in gas-filled radiation beam transport systems or in military search systems and particularly in high-quality optical systems exposed to ultraviolet radiation.